The present invention relates to novel brown spinel pigments based on zinc chromite which, in contrast to previously known pigments of this type, are iron-free. The production of the pigments of the invention is carried out in a manner which is generally known for spinel pigments but in which the baking temperature of the powder mixture is above the known range. The novel pigments can be used for ceramic decorations which can be fired or baked on in a high-temperature firing.
Spinel pigments based on zinc chromite are known; however, in many respects they do not meet the qualities desired by manufacturers of ceramic decorations. Zinc chromite of the formula ZnCr.sub.2 O.sub.4 is a spinel in which the Zn.sup.2+ ions occupy the tetrahedral positions and the Cr.sup.3+ ions the octahedral interstices of the cubically densest packing of oxygen atoms. ZnCr.sub.2 O.sub.4 is normally a greenish-gray product and is coloristically unattractive.
Spinels based on ZnO, TiO.sub.2 and Cr.sub.2 O.sub.3 are known from British patent 384,473. Such spinels are green. Spinels of this type can be produced by annealing a powder mixture of the oxides, the metals or precursors of the oxides at 800.degree. to 1100.degree. C. The powder mixture to be heat annealed can additionally contain known fluxing agents or mineralizers such as alkali halogenides or alkali sulfates.
Brown pigments are obtained by means of the insertion of iron into the spinel lattice of the zinc chromite. A disadvantage of these brown spinel pigments is their limited temperature stability in the presence of glass frits, which also applies to many other iron-containing spinels. Such pigments discolor during the firing on of decorative colors produced from them onto ceramic products with increasing firing temperature. Such pigments are therefore not satisfactorily suitable for applications which require firing temperatures above 1200.degree. C., especially above 1300.degree. C. to 1500.degree. C., such as e.g. new porcelain firing methods in which the unfired ceramic carrier and a decorative layer applied onto it are fired in a single firing cycle.